Daniel Moye's Technical Writings

Existence of high horizontal stresses in rock masses

Measurement of the deformations of the tunnels,
during construction, showed horizontal inward movements
of the rock walls greatly in excess of those predicted on
the basis of elastic behaviour of the rock influenced only
by its weight(12, 13). Also apparently the excavation of the
first tunnel had a marked effect upon rock movement in
the region of the second tunnel, even though they were so
far apart that this effect should have been negligible. In the
same area, inward movements of the rock walls of a canal
50 to 60 ft. deep were about one inch. All of these
observations suggest the existence of considerable
horizontal compressive stresses in the area.

Origin of the High Horizontal Stresses

In most of the examples quoted, from several widely
separated localities, the high horizontal stresses cannot be
explained merely as an effect of the weight of material, at
present overlying the sites. Surface topography is important
in some cases, but in others it is apparent that the
cause of the high stresses had to be sought in the geological
history and in the present state of activity of tectonic forces.

Except perhaps for the comprehensive measurements
made at some sites by Hast, the stress measurements
themselves are insufficient, and there is not enough
known about the geological history, rock structure,
and terrain, for a discussion of possible causes of the
high horizontal stresses to be more than very general
and speculative.

Some rocks, granites for example, at an early stage of
their history were probably at a sufficiently great depth to
be in a hydrostatic state of stress. Subsequent regional
uplift and reduction of the thickness of overlying rock by
erosion, would have permitted concurrent relief of vertical
compressive stress by upwards expansion of the rock. But
in the horizontal direction no such relief by expansion would
be possible (unless some geological process in addition to
vertical uplift occurred to permit it). The horizontal compressive
stresses would tend to decrease in accordance with
Poisson's ratio, but only at one-quarter to one-third the
rate of decrease of vertical stress. Thus the horizontal
stresses would become relatively higher than the vertical,
that is, N would become greater than 1. Rock creep, if it
occurred, would tend to equalize the stresses.

However, as already mentioned, many rocks exhibit
internal evidence, in the form of particular kinds of faults,
folds, and joints of having been subjected to high compressive
forces and it is considered that the effects of these
probably overshadow the gravitational effects in most cases.
Rocks compressed by these tectonic forces at some time in
the past would remain compressed in the horizontal
directions unless other crustal disturbances permitted them to
expand, or the compression was relieved by creep.